A wheel and a brake rotary member of a motor vehicle are supported rotatably on a suspension system by a wheel supporting rolling bearing unit. Since a large moment is exerted on a wheel supporting rolling bearing unit like this when the motor vehicle turns, rigidity against such a large moment or moment rigidity needs to be ensured in order to ensure the running stability. For this purpose, conventionally, as the wheel supporting rolling bearing unit, a construction has generally been used in which rolling elements are arranged in double rows and preload and a back-to-back type contact angle are imparted to the rolling elements in each of the two rows. For supporting wheels on suspension systems of motor vehicles, wheel supporting rolling bearing units of various types of constructions are known as are described in, for example, Patent Documents Nos. 1 to 6.
FIGS. 4 to 5 show a construction described in Patent Document No. 1 of those patent documents. A wheel supporting rolling bearing unit 1 of a first example shown in FIG. 4 of these figures includes a hub main body 3 and an inner ring 4 which make up a hub 2 which is an inside diameter side raceway ring member, an outer ring 5 which is an outside diameter side raceway ring member and a plurality of rolling elements 6, 6. A flange 7 which supports a wheel is formed at an outer end portion of an outer circumferential surface of the hub main body 3 of those constituent components. (Outer with respect to an axial direction means a side which lies transversely outwards when the bearing unit is assembled on a motor vehicle, which is a lower side in FIG. 3 and a left-hand side in FIGS. 4 to 7. On the contrary, a side which lies transversely centrally means inner with respect to the axial direction, which is an upper side in FIG. 3 and a right-hand side in FIGS. 4 to 7. This is true throughout the description.)
An inner ring raceway 8a for an outside row is formed on the outer circumferential surface at an intermediate portion and a small-diameter stepped portion 9 whose outside diameter is decreased are formed at an inner end portion of the hub main body 3. Then, the inner ring 4 having an inner ring raceway 8b for an inside row formed on an outer circumferential surface thereof is fitted on this small-diameter stepped portion 9 so as to make up the hub 2 above. An inner end face of this inner ring 4 is pressed by a clamping portion 10 which is formed by clamping to expand diametrically outwards a cylindrical portion formed at the inner end portion of the hub main body 3, so as to fix the inner ring 4 to a predetermined position on the hub main body 3. Double rows of outer ring raceways 11a, 11b are formed on an inner circumferential surface of the outer ring 5, and the respective rolling elements 6, 6 are disposed between the two outer ring raceways 11a, 11b and the two inner ring raceways 8a, 8b so that pluralities of rolling elements 6, 6 are provided in the two rows, respectively.
Next, in a wheel supporting rolling bearing unit 1a of a second example shown in FIG. 5, a hub 2a, which is an outside diameter side raceway ring member and has a flange 7a for support a wheel, is disposed around a pair of inner rings 4a, 4a, which is an inside diameter side raceway ring members and do not rotate when fitted outwardly to a support shaft (not shown). Then, pluralities of rolling elements 6, 6 are provided, respectively, between outer ring raceways 11a, 11b formed on an inner surface of the hub 2a and respective inner ring raceways 8a, 8b formed on outer circumferences of the respective inner rings 4a, 4a. 
In addition, FIG. 6 shows a construction described in Patent Document No. 5. A wheel supporting rolling bearing unit 1b of a third example includes a hub 2, an outer ring 5, and a plurality of balls 6, 6 which are individually rolling elements. In these components, the hub 2 is formed by combining a hub main body 3 and an inner ring 4. The hub 2 has a mounting flange, which supports a wheel and a brake rotary member and is formed at an axially outer end side of an outer circumferential surface thereof and double rows of inner ring raceways 8a, 8b, which are formed at axially intermediate portion and an inner end portion thereof, respectively. In these two inner ring raceways 8a, 8b, the diameter of the inner ring raceway 8a as an axial outside row is made larger than the diameter of the inner ring raceway 8b as an axial inside row. Distal end portions of a plurality of studs 12 are fixed to the mounting flange 7, so that a brake rotary member such as a disc and a wheel which makes up a road wheel are allowed to be fixedly supported on the mounting flange 7.
In the construction shown in FIG. 6, to make different the diameters of the two inner ring raceways 8a, 8b, an outer circumferential surface side inclined stepped portion 14 is formed at an axially intermediate portion on the outer circumferential surface of the hub main body 3 in a position which lies slightly further axially inwards than the inner ring raceway 8a of the outside row. The outer circumferential surface side inclined stepped portion 14 is inclined so that an outside diameter thereof decreases as it extends axially inwards. A small-diameter stepped portion 9 is formed at an axial inner end portion of the hub main body 3 which lies further axially inwards than the outer circumferential surface side inclined stepped portion 14.
Then, the inner ring 4 having the inner ring raceway 8b of the axial inside row on an outer circumferential surface thereof is fitted on this small-diameter stepped portion 9, and this inner ring 4 is pressed against a rising surface 13 which lies at an axial outer end portion of the small-diameter stepped portion 9. In this state, the inner ring 4 is fixedly connected to the hub main body 3. Both the two inner ring raceways 8a, 8b have a circular arc-shaped cross section (a generatrix shape) and their outside diameters decrease as they approach each other (as they extend towards an axial center of the hub 2).
The outer ring 5 has double rows of outer ring raceways 11a, 11b on an inner circumferential surface thereof and a connecting flange 15 which fixedly connects the outer ring 5 to a suspension system and formed on an outer circumferential surface. In these two outer ring raceways 11a, 11b, the diameter of the axially outside outer ring raceway 11a is made larger than the diameter of the axially inside outer ring raceway 11b. For this purpose, in the construction in FIG. 6, an inner circumferential surface side inclined stepped portion 16 is formed at an axially intermediate portion on the outer circumferential surface of the outer ring 5 in a position which lies slightly further axially inwards than the outside outer ring raceway 11a. The inner circumferential surface side inclined stepped portion 16 is inclined so that an inside diameter thereof decreases as it extends axially inwards, The two outer ring raceways 11a, 11b have a circular arc-shaped cross section (a generatrix shape) and their outside diameters decrease as they approach each other (as they extend towards an axial center of the hub 2).
The respective balls 6, 6 are provided rollingly between the two inner ring raceways 8a, 8b and the two outer ring raceways 11a, 11b so that pluralities of balls 6, 6 are disposed therebetween, respectively. In this state, a preload and a contact angle of back-to-back type (DB type) are imparted to the balls 6, 6 which are disposed in the double rows. Pitch circle diameters of the balls 6, 6 in the two rows are made different according to a difference in diameter between the inner ring raceways 8a, 8b and the outer ring raceways 11a, 11b. Namely, the pitch circle diameter PCDOUT of the respective balls 6, 6 (outside row) in the axially outside row is made larger than the pitch circle diameter PCDIN of the respective balls 6, 6 (inside row) in the axially inside row (PCDOUT>PCTIN). In the illustrated example, although the balls 6, 6 are used as rolling elements, if a rolling bearing unit for a heavy motor vehicle, tapered rollers may be used as the rolling elements.
Heretofore, the constructions of the wheel supporting rolling bearing units described in Patent Documents Nos. 2 to 6 in which the pitch circle diameters of the rolling elements in the two rows are made different are explained. In these constructions, the moment rigidity is made large by such an extent that the pitch circle diameter PCDOUT of the outside row can be made large, and this facilitates the design to realize improvements in running stability when the vehicle turns and durability of the wheel supporting rolling bearing unit. On the other hand, since the pitch circle diameter PCDIN of the inside row does not have to be made large, part (knuckle mounting hole) of the suspension system dose not have to particularly be increased in diameter. Consequently, even though this part of the suspension system is not particularly made large in size, the improvement in both running stability and durability can be realized.
While in the construction shown in FIG. 6, the diameters of the rolling elements 6, 6 which are disposed in the double rows are made equal, as with a wheel supporting rolling bearing unit 1c shown in FIG. 7, there has conventionally been known a construction in which the diameter of rolling elements 6a, 6a in an outside row is made smaller than the diameter of rolling elements 6b, 6b in an inside row. In this construction, as a fourth example, shown in FIG. 7, the number of rolling elements 6a, 6a in the outside row is made sufficiently larger than the number of rolling elements 6b, 6b in the inside row, whereby the rigidity of the outside row is made much higher than the rigidity of the inside row. Although the balls are used as the rolling elements in the respective illustrated examples, tapered rollers may be used as rolling elements for a rolling bearing unit for use on a heavy motor vehicle.
While any of the constructions shown in FIGS. 4 to 7 is automatically assembled on a production line of a bearing factory, if a defect such as a flaw exists somewhere in the construction, noise and vibrations generated while driving the vehicle and also a sufficient durability cannot be ensured. Thus, in a bearing factory, all or part of assembled wheel supporting rolling bearing units are inspected (a total inspection or a sampling inspection) to determine on the existence of defects. If there should be a defect, then, the production line is repaired or modified to eliminate a cause which generates the defect.
In general, a flaw on a rolling contact portion is considered as a type of defect which triggers a repair or modification of the production line. Namely, if flaws exist on the rolling contact portions between the rolling surfaces of the respective rolling elements 6, 6a, 6b and the respective raceways 8a, 8b, 11a, 11b, immoderate vibrations are generated when operating the wheel supporting rolling bearing unit and also early flaking originated from the flaws is generated, and then leading to a possibility of remarkably shorten the durability of this wheel supporting rolling bearing unit. Flaws which constitute a cause for the drawback like this are singly generated by a contamination of foreign matters or caused by a trouble on the production line side such as a failure in controlling the production facility. If such a trouble occurs, unless the production line is repaired immediately, plural defective products are produced, thus deteriorating the yield of products.
Thus, it becomes necessary to determine the existence of flaws on the rolling contact portions through inspection. As an inspection method for determining the existence of flaws like this, as is described in Patent Document No. 7, it is general practice to measure vibrations of a rolling bearing unit and to measure whether or not vibrations of a large amplitude (remarkably large vibrations compared with vibrations generated during normal driving) are contained in a specific frequency in the vibrations so measured. The revolving speed of the respective rolling elements which make up the rolling bearing unit (the rotational speed of the cages) is expressed bync=(ni/2)·{1−cos α/(dm /d)}when the inside diameter side raceway ring member rotates, and is expressed bync=(ne/2)·{1+cos α/(dm /d}when the outside diameter side raceway ring member rotates. In both the expressions, d denotes the diameter of the rolling elements, dm the pitch circle diameter of the rolling elements, α the contact angle [°] of the rolling elements, ni the rotational speed [s−1] of the inside diameter side raceway ring member and ne the rotational speed [s−1] of the outside diameter side raceway ring member.
For example, assuming that the number of rolling elements is Z, if a flaw exists in one location on an outer ring raceway in a circumferential direction thereof with an inside diameter side bearing ring rotating, vibrations in a frequency of Z·nc[Hz] are generated, while if a flaw exists in one location on an inner ring raceway in a circumferential direction thereof with the inside diameter side bearing ring rotating, vibrations in a frequency of Z·(ni−nc) [Hz] are generated. Vibrations in specific frequencies are similarly obtained when an outside diameter side raceway ring member rotates. Further, if a flow exists on the rolling surface of the rolling element, a frequency of vibrations based on the flaw is obtained based on the rolling speed of the rolling element itself.
Thus, by rotating the inside diameter side raceway ring member (or the outside diameter side raceway ring member) of the assembled rolling bearing unit, measuring vibrations of this rolling bearing unit and determining whether vibrations in the frequencies described above exist in the measured vibrations, whether or not a flaw exists on the rolling contact portions of this rolling bearing unit can be determined.
The determination in the way is implemented without any specific problem if the rolling bearing unit is a single row rolling bearing unit. However, if a double row rolling bearing unit like the wheel supporting rolling bearing units shown in FIGS. 4 to 7, it is not possible to know on which rolling contact portion of the row the flaw exists without any modifications to the flaw inspecting configuration for single row rolling bearing units. Namely, in the conventional wheel supporting rolling bearing units shown in FIGS. 4 to 5 which are used generally, since the specifications (the diameter, number, pitch circle diameter and contact angle of the rolling elements) of the two rows are the same, whether a flaw exists in the outside row or in the inside row, vibrations in the same frequency are generated. Therefore, it is not possible to specify in which of the rows the flaw exists, and hence, the repair of the production line cannot be started immediately. Namely, until disassembling the wheel supporting rolling bearing unit on which the flaw is determined to exist through analysis of the frequencies of vibrations to observe the raceway surfaces and the rolling surfaces, the row on which the flaw exists can not be specified. However, disassembling the wheel supporting rolling bearing is troublesome. In particular, in the constructions shown in FIGS. 4, 6, 7 in which the hub main body 3 and the inner ring 4 are fixedly connected together by the clamping portion 10, the clamping portion 10 needs to be ground off, and hence, time taken from confirmation of existence of the flaw to identification of the position where the flaw exists becomes long. When this time is taken long, time during which the production line is stopped to prevent the manufacture of defective products becomes long, and production efficiency becomes lower.
Further, in the wheel supporting rolling bearing units 1b, 1c shown in FIGS. 6 and 7, respectively, the contact angle of the balls 6, 6, 6a, 6b needs to be controlled properly as well as imparting a proper preload to the respective balls 6, 6, 6a, 6b so that the performances with respect to low torque characteristic, rigidity, durability and the like are exhibited as desired. It is general practice that the contact angle of the respective balls 6, 6, 6a, 6b is controlled to be equal (e.g., on the order of 20 to 45 degrees) between the outside row and the inside row. The larger the preload becomes, although the higher the rigidity of the wheel supporting rolling bearing units 1b, 1c, however, the higher the dynamic torque and shorter the fatigue life of the rolling surface. On the other hand, the larger the contact angle becomes, although the higher the axial rigidity, however, lower the radial rigidity and larger the spin at the rolling contact portion to thereby reduce the fatigue life of the rolling contact surface.
As is obvious from these facts, it is important to set the preload and the contact angle properly. Even in the wheel supporting rolling bearing units described in Patent Documents Nos. 2 to 6 in which the pitch circle diameters of the rolling elements in the two rows are made different, it is important to control the preload imparted to the respective rolling elements and the contact angle properly from the viewpoint that the performances with respect to low torque characteristic, rigidity, durability and the like are exhibited as desired.
However, in the wheel supporting rolling bearing units in which the pitch circle diameters of the rolling elements in the two rows are made different, since the rigidities of the two rows are different, unless a consideration, which is different from that to be taken for the general wheel supporting rolling bearing units in which the pitch circle diameters of the rolling elements in the two rows are equal, is taken, it is not possible to control properly the contact angles and the preloads of the two rows. The reason for this will be described below.
It is already known from, for example, Non-Patent Document No. 1 that when an axial load is exerted on a radial rolling bearing, the contact angle of respective rolling elements becomes large. In the wheel supporting rolling bearing unit which is intended by the invention, by imparting axial load to the rolling elements which are disposed in the double rows, a predetermined preload is imparted to the respective rolling elements. For example, in the construction shown in FIG. 6, the preload is imparted to the respective balls 6, 6, 6a, 6b which are disposed between the pair of inner ring raceways 8a, 8b and the double rows of outer ring raceways 11a, 11b by pressing the inner ring 4 outwards in the axial direction by the clamping portion 10 so as to shorten the pitch between the two inner ring raceways 8a, 8b. The magnitude of the preload (the quantity of the preload) becomes the amount of displacement of the inner ring 4 towards the outside in the axial direction from a state that the rolling surfaces of the rolling elements which are the respective balls 6, 6, 6a, 6b are brought into light contact with the two inner ring raceways 8a, 8b and the two outer ring raceways 11a, 11b (without imparting any preload to the respective balls 6, 6, 6a, 6b)
From this fact that imparting the preload to the rolling elements in the two rows (the balls 6, 6, 6a, 6b) in the wheel supporting rolling bearing unit which is intended by the invention is similar to the state in which the axial load is exerted on the radial rolling bearing, whereby the contact angle of the rolling elements in the two rows becomes large. In the general conventional wheel supporting bearing unit, since the specifications (the pitch circle diameter, the rolling element diameter, the number of rolling elements) of the two rows are equal to each other, amounts of changes in contact angle of the rolling elements in the two rows which occur in association with impartation of the preload are the same. Therefore, it has been relatively easy to control the preload and contact angle of the rolling elements in the two rows to proper values when the wheel supporting rolling bearing unit is completely assembled.
In contrast to this, in the wheel supporting rolling bearing unit which is intended by the invention in which the pitch circle diameters of the rolling elements in the two rows are made different, since the rigidities of the two rows are different, to control the preload to be imparted to the rolling elements (the balls 6, 6, 6a, 6b) in the two rows and contact angle properly, a special consideration is necessary. Namely, in the construction in which the pitch circle diameter of the axially outside row is larger than the pitch circle diameter of the axially inside row, the axial rigidity of the axially outside row becomes larger than the axial rigidity of the axially inside row. On the other hand, a force axially pushing the rolling elements in the two rows for imparting preload becomes equal, of course. Thus, when the inner ring 4 is pushed axially outwards to impart the preload, elastic deformation of the respective portions (the respective raceways and the rolling surfaces of the rolling elements) which is linked with impartation of the preload to the respective rolling elements becomes more on the outside row than on the inside row. As a result, degree at which the contact angle is increased in association with impartation of the preload becomes larger on the outside row than on the inside row.
Thus, in the wheel supporting rolling bearing unit which is intended by the invention in which the pitch circle diameters of the rolling elements in the two rows are made different, if the contact angles (initial contact angles) of the rolling elements in the two rows are made identical in a state that no preload has been imparted yet, as done on the conventional wheel supporting rolling bearing, the contact angles of the two rows become different after completion of assembling of the bearing unit (after the impartation of the preload). Furthermore, in association with generation of the difference in contact angle between these two rows, the preload imparted to the rolling elements in the two rows becomes different from the desired value (because the axial load that is generated in association with the axial displacement of the inner ring due to the impartation of the preloads is not generated as desired). As a result, the preload and the contact angle of the rolling elements in the two rows become improper.
In the construction shown in FIG. 6, while the construction is illustrated in which the inner ring 4 fitted on the axial inner end portion of the hub main body 3 is pressed by the clamping portion 10, the aforesaid problem is generated irrespective of the construction in which the inner ring is pressed or not. For example, although a construction is also known in which an axial inner end face of an inner ring is pressed by a nut which is thread fitted on an externally threaded portion provided on an axial inner end portion of a hub main body, the same problem is also generated in the construction like this. In addition, also in a construction in which an inner ring is pressed by a housing of a constant velocity joint, the problem is caused in a similar manner. Furthermore, even in the event that rolling elements are tapered elements, the same problem is generated.                Patent Document No. 1: JP-A-2004-142722        Patent Document No. 2: JP-A-2003-232343        Patent Document No. 3: JP-A-2004-108449        Patent Document No. 4: JP-A-2004-345439        Patent Document No. 5: JP-A-2006-137365        Patent Document No. 6: International Publication WO 2005/065077 pamphlet        Patent Document No. 7: JP-A-2004-361390        
Non-Patent Document No. 1: pages 62 to 65 of “Rolling Bearing, Dynamic Load Capacity of Rolling bearing,” written and published by Junzo Okamoto, and printed by Seibun-sha Ltd. Printing, in September, 1987